PCB's and other halogenated organic compounds are hazardous contaminants in soils, sludges, sediments and slurries. During the past three decades, studies performed on the toxicological effects of these compounds led to a ban on the use of PCB's and a ban, curtailment, or restrictions on the use of many other halogenated organic compounds. While the manufacture of PCB's is now prohibited in the United States, the size of the environmental burden in water, sediments, soil, disposal sites, and in electrical transformers is large. The National Academy of Sciences has estimated this burden at 1.25 billion pounds. For nearly 50 years, until the cessation of production in 1977, industry manufactured and used PCB's in the United States. The properties that made PCB's desirable for industrial applications, i.e., their chemical and thermal stability, as well as their non-flammability, are the same characteristics that make them a persistent problem in today's environment. The inherent thermal and chemical stability of PCB compounds also make them resistant to acid base reactions, hydrolysis, chemical oxidation, photo-degradation, and thermal changes. Today, PCB's are still found in electrical lighting ballasts, electrical transformers, and capacitors manufactured before the ban of PCB's in 1977. Moreover, as a result of manufacturing operations, spills and the disposal of electrical equipment, large areas of soil and sediment are also contaminated with PCB's. The contaminated material includes sediments and sludges in harbors, waterways, wetlands, and wastewater settling and discharge areas.
The U.S. Environmental Protection Agency has recommended a number of alternate treatment methods for PCB's. The most widely used method is incineration. Other methods include biological treatment, solidification, vitrification, treatment with potassium polyethylene glycolate (KPEG), solvent washing/extraction, and adsorption on granular activated carbon. Incineration is used for PCB contaminated soil, sediment, and liquids. However, it suffers from high cost and public resistance because of residues and stack emissions that may be contaminated with hazardous products of incomplete combustion or combustion by-products.
Two emerging technologies that are gaining acceptance include biological treatment and solvent washing/extraction. Biological treatment of PCB's is limited to relatively low PCB concentrations, may act very slowly, and may generate hazardous treatment by-products. Also, biological treatment has not been proven effective for all PCB congeners. Soil washing/extraction must be integrated with other disposal or treatment techniques such as incineration or other alternative dechlorination technologies such as KPEG. These techniques may have high cost and do not generally avoid the environmental and practical disadvantages of thermal or chemical destruction methods.
Ionizing radiation, i.e., x-rays/gamma-rays, electrons, or ions, has been shown to be an effective means of dechlorinating organic compounds. The chemical reactions induced by the ionizing radiation are called radiolysis. In 1974, Sawai, Shinozaki and Shimokawa Bulletin of the Chemical Society of Japan 1974, 47(8), 1889-93 reported the radiolytic dechlorination of PCB's in isopropanol and alkaline isopropanol. Subsequent investigations by Singh showed that in the presence of ionizing radiation, alkaline isopropanol solutions formed radical anions and solvated electrons. The radical anion and the solvated electron reacted with the PCB's in solution. These reactions led to the dechlorination of the compounds. In alkaline solutions, Singh also reported that isopropanol anions lose a proton to form an acetone anion. The acetone anion participates in the stepwise dechlorination of PCB's and produces acetone and biphenyls as the reaction products. Radiolytic dechlorination of PCB's in soil and oil matrices was proposed by Singh based on his experimental results.
In 1991, Mincher et al. Appl. Radiat. Isot. 1991, 42, 1061-1066 showed that stepwise dechlorination of PCB isomers such as 2, 2′, 3, 3′, 4, 5′, 6, 6′-octachlorobiphenyl at concentrations of 42 mg/l in neutral isopropanol solution occurs at applied gamma-ray dose between 20 kilograys (10 kilograys=10 kGy=1=megarad=10 joule/g absorbed energy) and 100 kGy. Mincher also reported that dechlorination of Aroclor 1260 (a PCB mixture) in electrical transformer oil is similar to the mechanism responsible for dechlorination in neutral solutions. Moreover, toxic oxidation byproducts such as dioxin and dibenzofurans are not generated by the reduction reaction in organic solutions. Based on these results, Mincher also proposed radiolytic dechlorination as a method of PCB destruction.
Although the radiolytic dechlorination of PCB's in solution has been well proven, the radiolytic dechlorination of PCB's in soil may require large doses. The large doses lead to higher cost for the treatment process. Although data on the radiolytic dechlorination of PCB's in soil is not presently available, recent research on dioxin (another hazardous halogenated organic compound)contaminated soil is available. Hilaride and Gray Environmental Progress 1994, 28, 2249-58 irradiated soil contaminated with 100 ng/l of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). In the presence of a surfactant (RA-40, 2%), with 25% moisture, and an applied dose of 800 kGy, approximately 93% of the TCDD was dechlorinated. Soil contaminated with the TCDD was also irradiated in the study. Approximately 55% of the TCDD was dechlorinated with 450 kGy of applied gamma-ray dose. Gray also reported that when electron beams were used instead of gamma-rays or x-rays from Bremsstrahlung sources, radiolytic dechlorination was not observed.
One example of a process for the decomposition of halogenated organic compounds is disclosed in U.S. Pat. No. 4,832,806 to Helfritch. The disclosed process directly irradiates the soil contaminated with the halogenated organic compounds. This process has the disadvantage of requiring large doses of radiation.
Several researchers have investigated solvent washing and extraction processes for recovering PCBs. Such processes can be used to extract the contaminants from the soil for radiolytic treatment of the contaminants without the interference of the soil provided scavengers. Kapila and Clevenger, at a field evaluation in Visalia, Calif., demonstrated an innovative soil washing flotation process for remediation of the soil in a batch process. Excavated soil containing dioxin and poly-cyclic aromatic hydrocarbon (PAH's) compounds from creosote were excavated and placed in processing bins. An alkane-alcohol mixture in a 5:1 ratio was then added to the soil. The alkane used in the experiments was SOLTROL™ 170 manufactured by the Phillips Petroleum Corporation. The alcohol used in the experiments was butanol, an alcohol with low water solubility. The amount of alkane-alcohol solution added to the soil was 28% by volume. This filled the pores of the soil. The alkane-alcohol mixture was floated out of the soil 12–36 hours after solvent incorporation. The removal efficiency for initial concentrations of 480–610 ng/kg of octachlorodibenzo-p-dioxin was well over 90%. Similar removal efficiencies for PAH concentrations of 630–5800 ng/kg were also reported. The PAH's included phenanthrene, fluoroanthene, pyrene, benzo-a-anthracene, benzo-b-fluoranthene, benzo-d-fluoranthene, chrysene, and dibenz-a-h-anthracene. Additional alkane-alcohol extractions were also shown to reduce further the concentration of contaminants in the soil. Once floated in water, the alkane-alcohol volume emulsified and could be easily separated from the flotation water. This reduced the volume of the contaminant (increased the concentration) by a factor of three.
Overcash et al. Environ. Sci. Technol. 1991, 25, 1479–85 had also shown a similar desorption process using isopropanol that could solubilize TCDD at slightly lower equilibrium concentrations. Partitioning of the TCDD off of the soil surface into the solvent was found to occur in 2–6 hours, typically, when alcohol alone was used as a solvent.
In other prior processes, the radiolytic dechlorination of Aroclor 1260 in electrical transformer oil was shown by Mincher. The results of Mincher and those of Gray's experiments suggest that if the soil or soil-like particles are not present, then the radiolytic dechlorination process would proceed efficiently. Moreover, in this case, the use of electron beams for radiolytic dechlorination of halogenated compounds can be economical.
In previous electron driven radiolysis practice, dose uniformity is achieved by low beam utilization or by ‘two-sided’ irradiation, i.e., the use of two opposing accelerators. In the case of solid objects to be treated, ‘two-sided’ irradiation can also be obtained by flipping the solid object over after treatment by an electron beam from one side and treating the opposite side of the object. Both of these approaches result in higher cost for treatment. Use of two accelerators at least doubles the size, complexity, and capital equipment cost of the facility. Flipping the target to be treated is most commonly performed on solid targets and has not been effectively done with multi-component liquids except in recirculating systems in which the material makes many passes. The lack of an inexpensive and easily implemented means to obtain dose uniformity has resulted in a higher cost of treatment. Accordingly, there is a continuing need in the industry for improved processes for treating contaminated soils containing halogenated inorganic compounds.